Thermoplastic materials are rapidly replacing glass and metal as the material of choice in many food packaging applications and many other applications which require a package or container which is highly impermeable to oxygen and other gases. Ethylene vinyl alcohol copolymers are known to be highly impermeable to such gases as long as the water content of the copolymers is relatively low. Ethylene vinyl alcohol copolymers (EVOH) are commonly used in packaging applications with polyolefins. The purpose of the polyolefins is to provide structural stability and also to prevent the passage of moisture into the EVOH, thus helping to maintain the oxygen barrier properties of the EVOH. Methods of making such containers include forming them from a sheet and injection blow molding. The sheet from which the container is formed must have a multilayer structure which provides a continuous oxygen barrier layer.
In order that they be able to achieve significant market acceptability, such packages or containers must be able to withstand heating without significant distortion. The ability to "hotfill"packages or to retort foods after they have been packaged permits the foods to be stored without refrigeration. Commercial sterilization by hotfilling or retorting imposes several additional restrictions on the choice of materials for the package: (1) the heat seal must survive commercial sterilization temperatures of over 180.degree. F. or typical retort conditions of steam or water at 250.degree. F. or more under pressure for one half hour or more; (2) the structure must not delaminate, unduly shrink or wrinkle as a result of the sterilization; (3) the oxygen and water barrier properties must not be permanently adversely affected by the conditions of commercial sterilization; and (4) the structure must be adequately strong to permit the handling of the package while still hot. The need for these additional hot filling or retorting requirements rules out many of the materials and structures commonly used for non-retorted film or sheet food packages.
Plastic food containers containing EVOH which are subjected to a steam retort process for food preservation will lose their oxygen barrier properties. This occurs because the polyolefins allow the passage of a great deal of moisture therethrough at the high temperatures of the retort process and the EVOH becomes saturated with water vapor and suffers a significant decrease in its oxygen barrier properties. As the EVOH later dries out over a period of days, the oxygen barrier is reestablished. However, because polyolefins are highly impermeable to moisture at normal temperatures, the drying out process takes a long time because a significant amount of water remains trapped within the EVOH layer. It is highly desirable to accelerate the removal of moisture from the EVOH to preserve the foods stored therein.